Why does my Decel/dashpot/declutch idle fall below DSDRPM

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dmills4229
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Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by dmills4229 » Sat Jul 27, 2019 11:32 am

My car is actually running and idling pretty well. I am just trying to sort out one particular behavior and I'm not sure where to start.

Car is turbo 331, H/C/I, 5-speed manual transmission. Using quarter-horse and BE.

My normal red stoplight approach is to gradually let off the gas, leaving the car in gear, and letting engine break begin to slow me down. As I get closer to the stoplight, I press the clutch in. At this point car has been engine breaking for at least 2-3 seconds. When I press the clutch in, rpms fall rapidly, and overshoots DSDRPM. The car usually recovers and RPMS bounce back above DSDRPM about 100 RPMs and then settles right on DSDRPM. But, if I happen to be making a right-hand or left-hand turn at the same time, the combination of turning (power steering engine loading) and overshooting DSDRPM has caused a stall before. Not the ideal time for car to die, lol.

It seems like if I change my driving behavior, and shift into neutral at the same time I let off the gas, rather than let the engine brake the car for 1-2 seconds, everything works fine. RPM falls to about DSDRPM + 400, and when I hit a complete stop, RPMs fall to DSDRPM.

I'm not sure if I understand when MINMPH, and DASMPH come into play, and if this is where I should be looking.

Any help in educating me on these functions would be awesome.

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by Paulie » Sat Jul 27, 2019 12:31 pm

Try increasing DASMIN. That is where dashpot decay will pause when you are above DASMPH. Once the vehicle speed goes below what DASMPH is set at the dashpot will start decaying again
1990 Mustang 5.0, HCI, Vortech S-trim, FRPP 42# inj., PMAS MH95, A9L, Moates Quarterhorse, BE/EA, Innovate LC-1.

dmills4229
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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by dmills4229 » Sat Jul 27, 2019 2:21 pm

OK cool. That value appears to be in lbs/min.. Is there a way I can correlate that to an RPM? Is that value as per the MAF function, or something else? I can just blindly increase it and see if it helps, but I would like to understand it.

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by Chucko » Sat Jul 27, 2019 3:11 pm

Paulie's right. The stall-on-declutch problem is pretty common in modified engines, and increasing DASMIN will fix it. Another way is to increase DASPTO, this is the approach I used initially. Two ways to achieve the same result.

Take a look at FN875N to find the correlation between lbs/min and RPM. But be prepared to experiment.
'89 LX 5.0, Crane 2040, Pro-M 75mm Bullet, 65 mm TB, ported stock intake & E7TE heads, 24 lb injs., JBA shortys, cat X-pipe, A3M1 (GUFB strategy) w/Moates Quarterhorse, BE & EA, close ratio T5Z, 3.55 rear

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by cgrey8 » Sat Jul 27, 2019 3:21 pm

I had this same problem because I like to activate DFSO which closes the ISC and cuts fuel. With the ISC closed, there's no DASPOT to decay. The ISC just has to open back up far enough and quick enough that it doesn't stall on declutch. However what I found that works really well is DNPPM which is an ISC adder that is added when the trans neutral switch goes from drive to neutral. I've got my scalar set to 4.4 lb/min.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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dmills4229
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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by dmills4229 » Sat Jul 27, 2019 3:36 pm

OK thanks for the ideas guys. Is there anything that you can think of in the strategy where declutching under a condition of engine-braking vs. declutching synchronously at closing the throttle changes things?

I am trying to wrap my mind around if declutching is declutching no matter if you are doing it at the same time as you lift off the throttle vs. doing it after the engine has been "dragging" for a few seconds.

Kinda like the difference between declutching using the practice of downshifting, vs. not downshifting.

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by cgrey8 » Sat Jul 27, 2019 5:51 pm

dmills4229 wrote:
Sat Jul 27, 2019 3:36 pm
OK thanks for the ideas guys. Is there anything that you can think of in the strategy where declutching under a condition of engine-braking vs. declutching synchronously at closing the throttle changes things? ...
Yes. During any decel, whether the clutch is in or not, the Preposition of the ISC is decayed down to a minimum. In the case of DFSO, the ISC may close completely and have to open to that minimum on declutch.

This is one of those areas of the strategy that I've always thought was lacking. I'm assuming newer code improved these scenarios.
dmills4229 wrote:
Sat Jul 27, 2019 3:36 pm
...I am trying to wrap my mind around if declutching is declutching no matter if you are doing it at the same time as you lift off the throttle vs. doing it after the engine has been "dragging" for a few seconds...
No there is a difference between those two scenarios...the daspot decay. The engine braking is not the difference. It's the passage of time between Closed Throttle and declutch. A far better strategy IMO would be to reset the ISC to Preposition on declutch, and begin decaying after the re-prepositioning. This would make declutching the same regardless of when it was done. But that's just not how the code is written.
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89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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dmills4229
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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by dmills4229 » Sat Jul 27, 2019 6:30 pm

No there is a difference between those two scenarios...the daspot decay. The engine braking is not the difference. It's the passage of time between Closed Throttle and declutch. A far better strategy IMO would be to reset the ISC to Preposition on declutch, and begin decaying after the re-prepositioning. This would make declutching the same regardless of when it was done. But that's just not how the code is written.

OK I think that is the light bulb moment for me! I wasn't thinking about the passage of time aspect . Thank you for that! I feel like a have some things to try now.

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by cgrey8 » Sat Jul 27, 2019 7:18 pm

With that, it should make more sense now why the DNPPM scalar adding ISC back to the equation on transition to neutral works. It's not exactly the same, but it's effectively/practically the same as prepositioning the ISC on the transition to neutral to give the decay logic something to decay again and thus avoiding the conk-out on declutch.
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by Chucko » Sun Jul 28, 2019 1:14 am

cgrey8 wrote:
Sat Jul 27, 2019 3:21 pm
I had this same problem because I like to activate DFSO which closes the ISC and cuts fuel.
That hasn't been my experience with DFSO. Several years of logs tell me it doesn't affect dashpot at all.
'89 LX 5.0, Crane 2040, Pro-M 75mm Bullet, 65 mm TB, ported stock intake & E7TE heads, 24 lb injs., JBA shortys, cat X-pipe, A3M1 (GUFB strategy) w/Moates Quarterhorse, BE & EA, close ratio T5Z, 3.55 rear

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Re: Why does my Decel/dashpot/declutch idle fall below DSDRPM

Post by cgrey8 » Sun Jul 28, 2019 9:04 am

Probably right...
...Always Somethin'

89 Ranger Supercab, 331 w/GT40p heads, ported Explorer lower, Crane Powermax 2020 cam, 1.6RRs, FMS Explorer (GT40p) headers, Slot Style MAF, aftermarket T5 'Z-Spec', 8.8" rear w/3.27s, Powertrax Locker, Innovate LC-1, GUFB, Moates QuarterHorse tuned using BE&EA

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